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Ra, Rz and Why Neither Describes How a Panel Looks

September 16, 2026

Surface roughness is one of the few appearance related things with a real number attached, which is exactly why it gets misused on cosmetic drawings. A number feels more rigorous than a reference sample. On a visible face it is usually less rigorous.

What Ra actually is

Ra is the arithmetic average deviation of the surface profile from its mean line, measured over a sampling length, in micrometres.

The key word is average. Ra sums up how far the surface wanders from flat and divides by the distance. That makes it excellent at describing a uniform texture and blind to a single defect.

Two surfaces can have identical Ra and look completely different:

• A uniform bead blasted face at Ra 1.6
• The same face at Ra 1.5 with one 20 micrometre deep scratch across it

The second one gets rejected by a customer and passes a roughness check. That is the whole problem in one example.

Rz measures the average of peak to valley heights, so it is more sensitive to individual features than Ra. It is a better number for cosmetic work than Ra, and it is still not a description of appearance.

Where roughness numbers are the right tool

Put Ra on drawings in these places and it earns its keep:

Feature	Typical Ra	Why
Transistor or device mounting pad	1.6 max	Thermal contact through the interface material
Gasket or O-ring sealing face	1.6 to 3.2	Too smooth and the gasket slides, too rough and it leaks
Bearing bore or sliding fit	0.8 or better	Friction and wear
Mating face that must sit flat	1.6	Contact, not appearance
General machined surface	3.2	The default, no extra cost
Face to be bonded	3.2 or blasted	Adhesion needs some texture

Each step tighter than 3.2 costs machine time. Specifying 0.8 across a whole part because it sounds high quality is a straightforward way to raise a quote for no benefit.

What to specify on a face people look at

Replace the roughness callout with a description of the process and an acceptance test. That is what actually controls appearance.

FRONT FACE:
  BEAD BLAST, GLASS BEAD #80-120 MESH, UNIFORM MATTE
  NO DIRECTIONAL PATTERN, NO PATCHINESS
  THEN ANODIZE TYPE II CLASS 2 BLACK, 10-15 MICRONS
  NO SCRATCHES, DENTS OR HANDLING MARKS VISIBLE AT 300 mm
  UNDER D65 DIFFUSE LIGHT, 10 SECONDS PER FACE
  APPEARANCE TO MATCH APPROVED SAMPLE 1042-A

That specifies the process, the uniformity, the defect standard, the viewing condition and the physical reference. A roughness number adds nothing to it.

If you want a number as well, use it as a secondary check on the machined surface before finishing, for example “machined face Ra 1.6 max before blasting”. That constrains the input to the finishing process, which is a sensible thing to control.

Roughness and the finishing process

A useful mental model: the machining sets the starting point and the finish sets what you see.

• Bead blasting raises roughness and makes it uniform and non-directional. It is a leveller of appearance, which is why it hides machining so well.
• Brushing produces a directional texture. Its Ra depends on grit, and two brushed panels with the same Ra can look different if the grain runs differently.
• Polishing lowers roughness toward a mirror, and a mirror shows every remaining defect. This is why polished parts are the most expensive to make acceptable.
• Anodizing largely follows the surface it is given. It does not rescue a poorly prepared surface.

The practical conclusion is that roughness on a cosmetic part is controlled by the finishing specification, not by a number on the machined face.

Measuring it, if you do

If a roughness number is on the drawing, somebody has to measure it, and the measurement has conditions of its own:

• Sampling length and cut off change the result. A roughness value with no cut off stated is ambiguous.
• Direction matters on a directional surface. Measure across the lay, not along it, and say so.
• Where on the part matters. A roughness callout applies to the surface it points at, so point it precisely.

For most audio parts this is a reason to keep roughness callouts on functional faces only, where a single number genuinely describes what you need.

The short version

• Use Ra on sealing faces, mounting pads, bores and bonded surfaces
• Use Rz rather than Ra if you must put a number on something with occasional defects
• Do not use either as the specification for a face people look at
• Specify the finishing process, the uniformity, the defect standard, the viewing condition and a physical reference sample instead
• Keep the general tolerance at Ra 3.2 and call out only what needs better

Frequently asked questions

Keep reading

• Which Aluminum Alloy to Use for an Anodized Audio Part
• Anodize Colour Matching Across a Batch and Across a Year
• How to Specify a Brushed Finish on an Audio Front Panel

This guide is part of Finishing Audio Parts: Anodize, Brush and Blast, one of four sections in the guide library.